The present invention relates generally to preparing pulp which, in turn, is used in papermaking machines to make paper and paperboard products. More specifically, the present invention relates to processing secondary fibre furnishes for the manufacture of paper and paperboard products.
It is known to use secondary fibre furnishes in papermaking machines. In order to process the secondary fibre furnishes into paper and paperboard products, it is necessary to remove contaminants from the secondary fibre furnishes. Such contaminants typically include broken glass, sand, metal wire, string, plastic and others. Although it is necessary to remove sufficient contaminants to provide a sufficiently pure pulp, it is desirable to minimize the amount of fibrous material that is removed with the contaminants.
Typically, the prior art processes for processing secondary fibre furnishes to pulp utilized a first step of conveying the baled furnish to a repulper for defibering while at the same time removing contamination from the stock. The stock is then treated to remove high specific weight contaminants and the stock is then sent through a secondary pulper.
It is also known in the art, to utilize in lieu of the secondary pulper a high consistency pressure screen. However, since the pressure screen does not induce deflaking, a good percentage of the stock must be rejected and treated through a deflaker before it is processed by an atmospheric screen for final contaminant removal.
These prior art repulping methods of processing secondary fibre furnishes suffer numerous drawbacks. Once the baled furnishes with all their abrasive and plastic contaminants are subject to repulping, it becomes difficult to separate these contaminants from the fibre because of their reduced form. This results in high process energy demands due, in part, to extensive hydraulic treatment of the slurry. Moreover, the prior art repulping process typically results in inferior end product cleanness due to the excessive recirculation of rejects in the various reject treatment stages that reduce the contaminants' size and thereby renders their removal difficult.
Furthermore, the typical prior art secondary fibre processes result in diluted final rejects that are difficult to dispose of either by burning or trucking to landfills. These prior art processes also result in increased mill effluent that contributes to the cost of operation and adversely effects the environment.
The prior art processes also frequently do not provide a satisfactory product. Because of the extensive reject recirculation loop, there is a high degree of degradation of the fibre.
Prior art processes have been limited to operation in the range of consistencies up to about four percent. At consistencies greater than four percent, raggers will not operate and bales cannot be submerged in a continuous process.
Moreover, with respect to the equipment utilized in the process, there are disadvantages when utilizing typical secondary fibre processing methods. The prior art processes usually result in increased cost for equipment maintenance due to the abrasive contaminants that are present in the furnishes in the repulping and coarse screening modules. Furthermore, the typical prior art processes for secondary fibre processing have a high cost of capital equipment, piping, and installation.
Accordingly, there is a need for an improved secondary fibre recycling process.